Over the past decade research from several laboratories has revealed two types of on-center retinal ganglion cells in the cat, X and Y cells (Enroth-Cugell & Robson, 1966). The general purpose of the research program to be outlined in this proposal is to assess the spatial characteristics of the excitatory and inhibitory mechanisms which underlie the responses of these two types of cells. The effect of variations of the size and location (in the receptive field) of unmodulated adapting stimuli upon the sensitivity in the receptive field center and receptive field surround of single cells in the optic tract will be examined.